Make field protected so that Watchable can be extended

So that you can have custom watchables that have very specific
behaviour, think `class NameWatchable extends Watchable`. Useful to have
access to `listeners` when you do this.

packages/element-web-module-api/src/api/watchable.ts

@@ -26,7 +26,7 @@ function isObject(value: unknown): value is object {
  * @public
  */
 export class Watchable<T> {
-    private readonly listeners = new Set<WatchFn<T>>();
+    protected readonly listeners = new Set<WatchFn<T>>();
 
     public constructor(private currentValue: T) {}